Return key does not display-message in index view

[TJ Hayes]

I am using mutt 1.9.4 on Ubuntu Budgie 18.04.

QUESTION: How can I have <Return> open/display the selected email message from the index view? 

I have this in my muttrc:

bind index <Return> display-message

However, hitting <Return> does not open the selected message from the
index. Mutt gives no feedback like "this key is not bound" or anything
like that. I can use <Space> to display-message, but I'd rather use
<Return>.

What I've checked:
(1) :exec what-key, then <Return>
Mutt tells me: Char = <Return>, Octal = 12, Decimal = 10

(2) <Return> is listed as bound to display-message, when I hit ? to view the keybinds.

(3) My <Return> key does function to do other things within mutt. 
When I change folders or select an attachment, <Return> does function just fine. <Return> is bound to select-entry in maps other than index.

Thoughts?

Thanks!
TJ